The increasing level of integration demanded of integrated circuits such as DRAMs has increased the need to transfer high-resolution patterns from a mask to a wafer to create circuits having very small features. Pattern-transfer apparatus using an electron beam are capable of the required high-resolution imaging.
FIG. 4 shows a conventional charged-particle-beam (CPB) pattern-transfer apparatus 40. An electron gun (not shown in FIG. 4) produces an electron beam EB that illuminates a mask 2. A CPB optical system 3 situated on an axis AX projects an image of the circuit pattern defined by the mask 2 onto a wafer 5. The mask 2 and the wafer 5 are retained by respective stages 1, 4 that are provided with respective CPB reference marks 6, 7. The CPB reference mark 6 is typically a small aperture that transmits the electron beam EB. The CPB reference mark 7 includes one or more areas that can reflect the electron beam EB.
To achieve high-resolution pattern transfer, the wafer 5 and the mask 2 must be precisely positioned with respect to each other. To this end, the wafer 5 is positioned with respect to the axis AX of the CPB optical system 3 by mechanically positioning the wafer stage 4 using the wafer-stage CPB reference mark 7. The wafer stage CPB reference mark 7 is illuminated by the electron beam EB, and the reflected (backscattered) electrons produced from such illumination are detected with a backscattered-electron detector 8 that produces an electrical signal used to align the CPB reference mark 7 with the axis AX.
In addition to the CPB reference mark 7, the wafer stage 4 can include optical reference marks. In such a configuration, an optical microscope can be used to perform a coarse alignment of the wafer stage 4 with the axis AX using the optical reference marks prior to the more precise alignment described above using the CPB reference mark 7.
After the wafer stage 4 is positioned as desired, the mask 2 is aligned with the wafer 5. The mask stage 1 positions the mask-stage CPB reference mark 6 near the axis AX to allow the electron beam EB to irradiate the mask-stage CPB reference mark 6. The portion of the electron beam EB transmitted by the mask-stage CPB reference mark 6 is reflected by the wafer-stage CPB reference mark 7 and detected by the backscattered-electron detector 8. Electron-beam deflectors (not shown in FIG. 4) scan the electron beam EB with respect to the wafer-stage CPB mark 7. The amount of deflection that produces a maximum output from the backscattered-electron detector 8 corresponds to an alignment of the mask stage 1 and the wafer stage 4 with each other. This deflection is recorded and used to compute deflections necessary for alignment during subsequent pattern-transfer operations. The alignment of the mask stage 1 with the wafer stage 4 need not be performed each time the wafer 5 is changed.
After alignment of the mask stage 1 with the wafer stage 4 is complete, the mask 2 and wafer 5 are aligned using respective CPB reference marks provided on the mask 2 and wafer 5. (These reference marks are not shown in FIG. 4.) The alignment procedure is similar to that used for aligning the mask stage 1 with the wafer stage 4 using the respective CPB reference marks 6, 7. The deflection of the electron beam EB required to properly align the wafer 5 and the mask 2 is recorded and used to compute the electron-beam deflection and rotation required during subsequent pattern-transfer operations.
The conventional mask-to-wafer alignment method summarized above is adequate for small masks, but with larger masks, the conventional method is unsatisfactory. The dimensions of a large mask are less accurately controlled than those of a small mask so that the location of the mask-stage CPB reference mark 6 relative to the mask 2 is less accurately known. In addition, because the area illuminated by the electron beam EB is small, CPB reference marks can be outside the illuminated area so that alignment based on backscattered electrons from these reference marks is impossible.
In addition, for thin-film electron-scattering masks, the locations of CPB reference marks can change during use. The electron beam EB heats the mask, causing a thermal deformation of the mask. To reduce thermal deformation of the mask, an additional thin film can be deposited on an area surrounding the CPB reference mark to reduce heating. The additional thin film, however, also reduces the positional accuracy of the CPB reference marks. Thus, precise alignment is difficult.